1. Field of the Invention
The present invention is related generally to rotary mining and oil well drilling bits and more specifically to an improved bushing mounting structure for mounting and lubricating a rotatable cutter head on a mining or oil well drilling bit.
2. State of the Prior Art
"Rock bits" that are used in the mining industry to drill holes into rock formations and in the oil and gas industry to drill oil and gas wells into oil or gas bearing rock formations deep in the ground typically comprise a plurality (usually three) conical-shaped rotary cutter wheels that are rotatably mounted in a cluster on the distal end of a drive shaft or string of drill pipe. Each of such rotary cutter wheels usually has a plurality of hard radially protruding teeth that are designed to mesh loosely with teeth on adjacent cutter wheels and are oriented in such a way that, as the cluster is rotated about a major rotation axis by a drive shaft or drill pipe string, the teeth on the cutter wheels engage the rock formation into which a hole is being drilled and cut, break, or crush chunks or pieces of the rock formation so that such chunks or pieces can be carried out of the hole by a circulating drilling fluid.
The axial and angular forces that have to be applied to rock bits in order to achieve the rock cutting, breaking, and crushing action that is necessary to drill holes in rock formations are tremendous. The rock cuttings are hard and abrasive, and the resulting wear and tear on the rotary cutter wheels, especially in the journal mounting structures that rotatably mount the cutter wheels to the main body or trunk of the rock bits, are severe. There have been many improvements in all components of rock bits over the years, including, but certainly not limited to, rotary cutter mountings, lubricating systems, materials, teeth structures, drilling fluid nozzles, and the like. The numbers and varieties of such improvements and innovations are far too numerous to chronicle here. Yet, because of the large forces and severe conditions into which the rock bits operate, rapid wear and resulting breakage of cutter wheels and mounting component continues to be a constant and persistent problem.
The U.S. Pat. No. 4,572,306, issued to D. Dorosz, which is incorporated herein by reference, discloses a segmented bushing that is shrink-fit and further retained by a lock ring in the cutter wheel and rotatably mounts the cutter wheel in journal fashion on a spindle. It also discloses a second bushing and thrust surface around a protruding distal end of the spindle, a lubrication system for routing grease to the bushings, and an O-ring elastomeric seal at the proximal end of the spindle to keep abrasive rock debris away from the bushings. This rock bit structure has performed quite well as compared to other state-of-the-art rock bits for many years. However, failures of the seals and shortly thereafter bushing failures still occur too frequently. When the rock bit is on the end of a string of oil well drilling pipe that may extend one to two miles or more into the ground, it takes many hours to "trip" out of the well hole to get the rock bit to the surface where it can be changed and then many more hours to trip back into the well hole to resume drilling operations. If the cutter wheel mounting has failed badly enough to allow the cutter wheel to separate from the spindle, that rotary cutter wheel may be left in the bottom of the well hole when the rest of the rock bit is pulled to the surface. In such instances, other time-consuming and costly procedures must be undertaken to fish the lost cutter wheel out of the well hole, because it is made of very hard metal alloys and would inhibit a new rock bit from boring farther into the rock formation. The problem is compounded if the cutter wheel is lost in a horizontal well hole, because conventional fishing techniques and tools that are used in vertical well holes do not work as well, and some not at all, in horizontal well holes.